Laser technology has received considerable attention from the industry as a means to produce permanent markings on various substrates. Presently, laser technology makes use of a focused energy source to "burn off" some of the surface material to produce an image. Carbon dioxide lasers, with a wavelength in the infrared region, are commonly used for this application.
There are at times advantages in using the laser marking technology over other marking technologies, e.g., ink jet printing technology. For example, the maintenance of a laser equipment may be easier and more economical in certain circumstances than the maintenance of other types of markers. For examples, since laser technology does not depend on using a liquid ink jet to produce an image, laser technology is less prone to printing problems caused by ink. In addition, the laser technology allows marking of substrates at extremely high speeds. An example of the use of the technology is in the marking of expiration dates on plastic soda bottles. The rate of movement of the conveyor belt carrying the soda bottles in this application ranges from 100 to 300 feet per minute, and it can be as high as 500 feet per minute.
Although laser technology has several advantages, substrates suitable for laser marking are presently limited. For example, it is difficult to mark metals and white or corrugated papers. The metal surface or the white paper surface is too reflective for the laser energy to be converted to thermal energy which is needed to create a mark. In the case of corrugated paper, the laser tends to damage the surface. Thus, there exists a need for substrates or substrate surfaces that are suitable for laser marking.
There have been reports on a coating which is writable by laser. An example is given in DE 19652253-A1. A disadvantage of this coating is that it helps to produce only white markings on a colored substrate or background. In actual practice, it will be more desirable to have markings of brown or black over a white or colorless background. Thus, there exists a need for a substrate or surface that is suitable for creating brown or black markings on a white or colorless background.
These objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.